The present invention relates to a printer using a thermal printer head, and more particularly, to a heating method for minimizing changes of temperature and voltage of the thermal printer head and a heat control apparatus adopting the same.
The thermal printer head (TPH), which is a core of a thermal transfer type printer, consists of a plurality of heating elements and generates heat corresponding to gradation of a pixel to be printed. The generated heat is transferred to a thermosensitive recording sheet which runs in contact with TPH so that an image is printed.
TPH has heating and non-heating states corresponding to binary data of "0" and "1". To represent various gradations with TPH, TPH must be repeatedly actuated as many as the value of the gradation required.
For instance, when a gradation value of a pixel is "55," the heating element of TPH must be repeatedly heated 55 times with respect to the pixel.
FIG. 1 shows a conventional TPH heating controlling apparatus. In the drawing, a reference numeral 10 denotes an address generator; a reference numeral 12 denotes a line memory; a reference numeral 14 denotes a gradation generator; a reference numeral 16 denotes a comparator; and a reference numeral 18 denotes a line buffer.
Line memory 12 is for storing "n" units of m-bit data, and one vertical line of the image data is stored therein. Here, the one vertical line of the image data means a group of pixels corresponding to an arbitrary one vertical line among groups of pixels (vertical lines) arrayed in a column direction, when a frame image is digitally converted and arrayed in a matrix form.
Since the pixel data stored in line memory 12 is m-bit, the pixel data can have gradation values of 0 through 2.sup.m -1.
Gradation generator 14 is for successively generating each gradation value from 1 through 2.sup.m -1. The gradation value generated from gradation generator 14 is successively compared with the one vertical line of the data stored in line memory 12.
Address generator 10 for generating an address which designates a pixel read out from line memory 12 successively generates addresses from the lowermost address to the uppermost address. Also, address generator 10 actuates gradation generator 14 to shift the gradation value.
Comparator 16 is for comparing the gradation value generated from gradation generator 14 with the pixel data read out from line memory 12 and outputting "1" when the pixel data is greater than the gradation value and "0" in the reverse.
Line buffer 18 is for storing the compared result of comparator 16 by pixel. The contents stored in line buffer 18 are used for actuating TPH.
FIG. 2 shows a flowchart for explaining a method of printing the one vertical line by using the apparatus shown in FIG. 1.
First, in step 200, a gradation value generated from gradation generator 14 is renewed. The renewed gradation value is a consecutively increased value, relative to the previous gradation value.
In step 202, the pixel data stored in line memory 12 is read out from the uppermost pixel to the lowermost pixel. For this reading, address generator 10 generates addresses successively increasing from the uppermost address for designating the uppermost pixel down to the lowermost address for designating the lowermost pixel.
In step 204, the gradation value generated from gradation generator 14 is compared with the read out pixel value through comparator 16.
In step 206, the comparison result of step 204 is stored in line buffer 18.
In step 208, after all the pixel data of line memory 12 is read out and the gradation comparison completed, address generator 10 sends a gradation shift strobe signal for requiring a gradation shift to gradation generator 14. TPH is heated according to the comparison result stored in line buffer 18, to print one vertical line of the pixel data.
In step 210, it is judged whether all the gradation values have been generated in gradation generator 14. If all the gradation values are not generated, the operation returns to step 200. The completion of generating the all gradation values in gradation generator 14 means that printing for one vertical line is over.
Through the heating process described in FIG. 2, printing of one vertical line is performed. When the printing of the one vertical line is completed, the next vertical line data is stored in line memory 12. By repeating such a process with respect to all the other vertical lines, the printing of one frame image is accordingly performed.
Here, it is taken as an example that the gradation value generated from gradation generator 14 is generated in an order of 1, 2, 3, . . . , and (2.sup.m -1). However, there is another method for generating the gradation value by separating the same into odd numbers and even numbers, that is, 1, 3, 5, . . . , and (2.sup.m -1), and 2, 4, 6, . . . , and (2.sup.m -2).
FIGS. 3A and 3B show distribution of heat amount of TPH with respect to a time axis in the apparatus shown in FIG. 1. In FIG. 3A, the gradation value is in an order of 1, 2, 3, . . . , and (2.sup.m -1), and in FIG. 3B, the gradation value is in an order of 1, 3, 5, . . . , and (2.sup.m -1), and 2, 4, 6, . . . , and (2.sup.m -2).
In the apparatus shown in FIG. 1, the gradation value is successively shifted, and thus, the heat is intensified around a low gradation portion with respect to the time axis.
As shown in FIGS. 3A and 3B, since the heat is relatively great in the low gradation portion, a heat accumulation phenomenon becomes serious while the gradation value generated in gradation generator 14 is respectively lower. Thus, since the temperature of TPH rises, the TPH function is totally deteriorated.
Further, since most heating elements of TPH become a heating state in the low gradation, a TPH driving voltage applied to each heating element in common is lowered, affecting the heat amount of TPH.